Relays



Feb. 18, 1958 Filed Feb. 25, 1955 FIG] H. K. KRANTZ RELAYS 5Sheets-Sheet l lNl/ENTOR H. k. KRANTZ ATTORNEY RELAYS 5 sheets-sheet 2Filed Feb. 25, 1955 FIG. 2'

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lNVENTOR h. K. KRANTZ ATTORNEY Feb; 18, 1958 H. K. KRANTZ 2,824,266

RELAYS Filed Feb. 25, 1955 5 Sheets-Sheet 3 INVENTOR y H. K. KRANTZL4TTORNE Va Feb. 18, 1958 H. K. KRANTZ RELAYS 5 Sheets-Sheet 4 FiledFeb. 25, 1955 FIG. 7

FIG.8

INVENTOR H. A. KRANTZ ATTORNEY Feb. 18, 1958 H. K. KRANTZ 2,824,266

RELAYS Filed Feb. 25, 1955 5 Shets-Sheet s FIG. .9

PULL CURVE OF FLEXIBLE ARMATURE W/T H CUP CORE PULL IN GRA MW PULL CURVEOF 40 INFL EX/BLE I ARMATURE WITH FLA r 5' CORE 0 l I I I I I l I I I IL O5IOI520253035404550555O ARMATURE CLOSED ARMATURE A EL- M|| 5Ai-EMJAT'UAE OPEN F 16. IO

I009 (I) E CUP CORE' W/TH C! f VARIATIONS IN FLEXIBILITY o OFARMATURE Zs J PULL CURVES 500 3 375 0 l I II I I 'IO 20 7 3O 4O 6k APMATURE CLQFEDARMATURE TRAVEL MH ARMATUFE OPEN INVENTOR H K. KRANTZ ATTORNEY UnitedStates Patent i RELAYS Hubert K. Krantz, Rockville Centre, N. Y.,assignor to Bell Telephone Laboratories, Incorporated, New York, N. Y.,a corporation of New York Application February 25, 1955, Serial No.490,567

Claims. (Cl. 317 165) This invention relates to electromagnetic devicesand, more particularly, to magnetic systems for relays, especially forrelays of the wire spring type such as disclosed, for example, in Patent2,682,584, granted June 29, 1954, to H. M. Knapp.

In general, the magnetic system in relays of the type above mentionedcomprises a core having a first leg or pole-piece and a secondpole-piece having one or more legs spaced from the first pole-piece, thelegs being substantially flat and parallel. A coil is coupled to thecore and is effective when energized to magnetize the two polepieces toopposite polarities. Mounted for pivotal movement toward the pole-piecesin response to energization of the coil is a rigid armature which, inthe operated position, substantially bridges the pole-pieces.

Among the operating characteristics of particular moment in theevaluation of a magnetic system of the character above mentioned are thepull characteristics, e. g. the relation between the load and the forceeffective upon the armature, the operate times, closing force and,concomitantly therewith, relay chatter, and, of course, efficiency.

In known constructions, the effective pole face area is substantiallyconcentrated whereby a fixed lever ratio during movement of the armatureobtains. As the armature approaches the pole-pieces, the pull thereonincreases continually so that substantially maximum force is exertedupon the armature at the end of its travel. This is conducive to contactchatter and also to excessive wear. Further, in known constructions, thereturn flux path areas of the core contribute little, if at all, to thepull effective upon the armature so that the efiiciency is relativelylow. Moreover, in known constructions, efiiciency is degraded by virtueof flux leakage.

One general object of this invention is to improve the performancecharacteristics of electromagnetic devices.

More specifically, objects of this invention are to realize improvedpull characteristics in electromagnetic devices such as relays, toreduce chatter and Wear in such devices, to decrease the operate andrelease times, to permit the use of thinner and lower grade magneticmate rials and to increase the efficiency.

In accordance with one feature of this invention, in a magnetic systemof the character discussed hereinabove, the pole-pieces and armature areconstructed and arranged to provide a pull characteristic of prescribedconfiguration. More specifically, in accordance with one feature of thisinvention, the pole-pieces and armature are constructed andcooperatively associated to provide a pull characteristic conforming toor matching the load curve for the armature.

in one illustrative embodiment of this invention, the magnetic system ofa wire spring relay comprises a core having a pair of coaxial,substantially coplanar pole faces and an armature mounted for rockingmovement relative to the pole faces. The armature advantageously isfiat, may be of polygonal configuration and is of transverse dimensionsat least equal to those of the outer pole face. It

2,824,266 Patented Feb. 18, 1958 2 is mounted normally inclined withrespect to the pole faces by a support laterally beyond one side of theouter pole face. Thus, the armature forms an air gap with both polefaces, the gap between the armature and the outer pole face increasingin length away from the support. Upon energizaticn of the coil,initially a large force or pull is exerted upon the armature adjacentthe portion thereof for which the air gap aforementioned is relativelysmall, this force acting over a relatively short lever arm. As thearmature moves toward the pole faces, the effective pole face areavaries progressively so that as the armature approaches the end of itstravel, the force or pull thereon increases only slightly at most andacts over a relatively long lever arm. Consequently, a comparatively lowE final armature velocity is achieved and, in the case of a relay,closing impact shock of the contacts is reduced. Further, it has beenfound, this construction enables enhancement of the operate and releasetimes.

The pull characteristic above described approximates, generally, theload upon the armature. This characteristic can be tailored to match theload, particularly effectively through control of the stiffness of thearmature. In especially advantageous constructions, the armature is madesufficiently flexible to bow concave upwardly relative to the pole facesupon initial energization of the coil. In such construction, inoperation, the armature executes a rolling-like motion over or relativeto the pole faces. By controlling the stiffness of the armature, thepull characteristic can be made such that the change in force effectiveupon the armature throughout the end por tion of its travel is minimal.

All of the features of the invention will be readily understood from thefollowing detailed description when read with reference to theaccompanying drawings in which:

Fig. 1 is a plan view of a relay, a preferred embodiment of theinvention, partially cut away to disclose the pole faces, terminalarrangements and other features of construction;

Fig. 2 is a longitudinal section of the relay shown in Fig. 1 takenalong the line 2-2 and looking in the direction of the small arrows;

Fig. 3 is a transverse section of the relay shown in Fig. 1 taken alongthe line 3-3 and looking in the direction of the small arrows;

Fig. 4 is a back elevation of a part of the relay shown in Fig. 1 takenalong the line 4.4 and looking in the direction of the small arrows;

Fig. 5 is an exploded view of the major components of the magneticstructure of the relay shown in Fig. 1;

Fig. 6 is a schematic cross section of some of the major components ofthe magnetic structure of a relay of the general type shown in Fig. land illustrates particularly the configuration of a stiff armature atthe instant after energization of the coil but prior to separation ofthe armature from its backstop;

Fig. 7 is a schematic cross section of some of the major components ofthe magnetic structure of a relay of the type shown in Fig. l andillustrates particularly the configuration of a flexible armature at theinstant after en: ergization of the coil but prior to separation of thearmature from its backstop;

Fig. 8 is a schematic View of another embodiment of the inventionemploying a deep W type core structure with the armature transverselydisposed with respect to the coil and core structure;

Fig. 9 is a graph of a family of load curves of a relay of the wirespring type together with the pull curve of a stiff or inflexiblearmature associated with a conventional E or flat core compared with thepull curve of a flexible armature operatively associated with a corearrangement constructed according to the invention; and

Fig. 10 is a graph of a wire spring relay load curve together With afamily of flexible armature pull curves and illustrates particularly theflattening effect produced on the pull curve by increasing armatureflexibility.

Referring now to Figs. 2 and 5, illustrating the magnetic structure ofthe relay of Fig. 1, it will be noted that the major elements includethe armature 4, 'the armature spring 5, the non-magnetic spacer 3, thecoil 2, and the core 1. The armature 4-, when in the operated or closedposition, bears against the non-magnetic spacer 3 which in turnseparates the armature 4 from the outer pole face 2-9 and from the innerpole face 41. The coil 2 fits into the annular ring of the core 1. Thecore it can be manufactured advantageously from a single drawn orstamped piece. It will be noted further that except for the top surfaceof the coil 2, the coil is entirely contained within the corestructure 1. This containment of the coil 2 by the core structure 1results in a magnetic structure of small vertical dimension. Inasmuch asthe dimension of the magnetic structure is the primary determiningfactor affecting the size of the relay, this feature is particularlydesirable in relays employed in multiple banks where spaceconsiderations are of importance. Moreover, a further distinct advantageis gained by the association of the core and coil in the mannerdescribed. Containment of the coil by the core results in a large areamagnetic circuit of low reluctance which reduces leakage flux to aminimum. As a result, fewer ampere turns are required for successfuloperation. Included among the advantages associated with reduced ampereturns is the fact that there is less heat to be dissipated, the core andarmature can be manufactured of relatively low grade magnetic materialsuch as cold rolled steel and also those components can be made thinnerand lighter than is the case in a relay in which the core is essentiallyflat.

The manner in which the armature t is supported and pivoted may best beunderstood from Figs. 2 and 5. The pivoting side of the armature 4 isextended in two legs 35 which are spot-welded to the correspondingsections of the armature spring 5. It will be noted that the top portionof the armature spring 5 is divided into five sections by means of fourslots 42 in order to attain a pre- 7 scribed degree of flexibility. Thetop normally horizontal portion of the armature spring 5 rests against aphenolic supporting piece or spacer 9. The spring 5 and armature legs 35are secured in position between the spacer 9 and a second phenolicpiece, the twin wire block 8. The twin wire block 8 has recessed areasto accommodate the armature legs 35. The spacer 9 rests on the mountingbracket 10. The bottom member of the pileup or armature supportstructure is the rear single wire block 11. The pileup is secured inplace by means of a top clamp plate 13, a bottom clamp plate 12 and fourstaples 14, each of which passes through the bottom clamp plate 12, therear single wire block 11, the mounting bracket 19, the spacer 9, thearmature spring 5, the twin wire block 8, and finally the top clampplate 13. The eight holes 43 in the armature spring 5, which aredesigned to receive the four staples 14, are illustrated in Fig. 5. Fromthe foregoing description of the armature support structure, it will beunderstood that the armature is pivoted at a point laterally beyond theouter pole face 40, thus forming an air gap with both. the outer poleface 40 and the inner pole face 41. Further, it will now be apparentthat in the unoperated position, the air gap between the armature andany given point on either pole face will vary directly with the distanceof that point from the pivot or hinge axis of the armature.

Referring now to more of the details of the exemplary embodiment of theinvention, illustrated in Figs. 1, 2, 3, 4 and 5, it will be noted thatthe contact end of the armature 4 is extended on each side so that thearmature assumes a T configuration. Mounted across the end of thearmature 4, on the cross of the T, is a non-magnetic twin wire guidecomb bar 6 with slots provided to hold the top twin wire leads in place.The guide comb bar 6 is held in place against the armature 4 by a metalspring clip 7 which is in turn spot-welded to the armature 4. The radialsectorial apertures in the armature 4 are designed to lighten thearmature and to control its stiffness. Also, the apertures are spacedand proportioned to give the armature the desired magnetic fluxconductivity.

The squared notches 34- in the outer pole face of the core are designedto receive the two tongues of the separator 3 which are bent at rightangles to the plane of the pole face and thus retain the separator 3snugly against the pole faces 4% and 41 of the core 1. The roundednotches 33 in the outer pole face an of the core 1 are designed topermit insertion of the helical back tension springs 25, best viewed inFig. 3. Operation of these springs will be explained later herein. Thesquared projection 52 from the outer pole face of the core 1 is thearmature backstop and is designed to engage the armature backstop catch31. Upon deenergization of the magnetic circuit, the armature pivotsaway from the pole faces, forced by the action of the armature spring 5and the helical bacl; tension springs 26, until the inner lower portionof the armature backstop catch 31 rides against the lower portion of thearmature backstop 32.

As noted heretofore, the upper or movable set of relay contacts 16 arecarried on the ends of pairs of conventional sets of wire spring leads15. The paired spring leads are iaintained in position at the back ofthe relay by the twin wire block 3 which is composed of laminatednon-magnetic sections which enclose approximately one-third of thelength of the twin wires 15.

Tracing the path of the movable sets of twin wire leads 5.5 from theterminal arrangements at the rear of the relay and passing through thetwin wire block, it will be note that the wires are bent slightly topretension them to create a downward force to hold the wires in theirrespective slots in the twin wire guide comb bar 6. Continuing to tracethe circuit, Fig. 2 illustrates the relay 1n the open or unoperatedposition with the open circuit gap appearing between the twin wirecontacts 16 and the fixed single wire contacts 17. Each of the fivegroups of sur single wire leads passes through a laminated phenoliccontact adjustment and alignment block 1?. Proper alignment of each ofthe five groups of single wire leads and contacts is attained byexerting a force against the appropriate alignment block 19 in order toimpart the desired degree of bending stress against the group of singlewire leads. After passing through the contact adjustment and alignmentblocks 19, the single wire leads are bent at right angles to passthrough the front single wire block 20, continuing back under themounting bracket 10, through the rear single wire block 11 and on intothe rear single wire terminal arrangement 36.

Returning now to the front single wire block 2t), as illustrated in Fig.2, it will be observed that two metal clips 21 are secured by staples 22to the underside. The clips 21 are firmly tensioned against theunderside of the front single wire block 20 in order to hold arectangular box-like plastic contact cover 33 which may be fitted overthe entire front or contact end of the relay in order to keep thecontacts free from dust or other foreign matter. The plastic cover 38has a groove on the outside of each side which accepts the inside edgeof each of the two front projecting prongs or cover guides 37 of themounting bracket 10. The plastic cover 38 has four plastic sections ordivisional walls each of which fits into one of the four spaces betweenthe contact adjustment and alignment blocks 1?. The plastic divisionsextend back to a depth which is approximately equal to the thickness ofthe contact adjustment and alignment blocks 19. The same staples 22which secure the cover clamps 21 to the front single wire block 20 passthrough the mounting bracket 14) and through the two spring adjustmentplates 23.

The spring adjustment plates 23 perform a dual function. They serve aslower bearing surfaces for the helical back tension springs 26 and alsoprovide a means for adjusting the helical back tension springs 26 to thedesired tension. Each back tension spring 26 is fitted over the smallright cylindrical projection 24 or centering shoulder of the springadjustment plate 23. The top of each back tension spring fits around andis centered by a small right cylindrical projection, not shown, on thebottom of the front section of the armature 4. The armature 4 thusoperates against the tension of the armature spring 5 and alsocompresses the helical back tension springs 26. Adjustment of thetension of the helical back tension springs 26 is effected by bendingthe spring adjustment plates 23. This bending may best be accomplishedby exerting force on any small pointed tool inserted in the holeprovided in the bent over tip 25 of the spring adjustment plate 23.

Introduction of current into the coil 2 is effected by means of two coilsupply contacts 28, one of which is visible in Fig. 2 extending belowthe core 1. Current to the contacts 28 is supplied by two coil supplyleads 29, illustrated in Figs. 2 and 3. Each coil supply lead bearsagainst the appropriate coil lead contact 28 and parallels the severalsingle contact wires 13 running under the relay mounting bracket in andthrough the rear single wire block 111. The terminal end of a coilsupply lead 29 is illustrated in both Figs. 2 and 4.

The foregoing descriptions of the armature 4 have not includeddiscussions disclosing its unique flexibility or bendingcharacteristics, noted above as a significant feature of the invention.Both the theoretical and practical aspects of this feature will bereadily understood by reference to Figs. 6, 7, 9 and in connection withthe following discussion.

Traditional practice in the design of armatures for conventional relayscalls for a rigid or stiff armature which remains substantially flatwithout bending while carrying a load during its period of travel. Fig.6 schematically illustrates a conventional stiff armature 4 incombination with a cup-type magnetic core structure 1 integrallyassociated with concentric pole faces forming an essentially annularreceptacle for the insertion of the coil 2. In Fig. 6, current has beenintroduced to the coil 2, fiux has started to build up and exert a pullon the armature 4, but insufiicient time has elapsed to overcome theopposing spring tension and hence the armature still rests against itsbackstop 37. It should be noted that with the type of relay illustratedin Fig. 6, the armature 4 will remain essentially flat and rigidthroughout its entire arc of travel from its position of rest againstits backstop 37 to its closed position in a plane essentially parallelto the pole faces.

in contrast to Fig. 6, Fig. 7 illustrates the action of a flexible ornon-rigid armature 4. In other respects, the general construction of therelay is the same as that illustrated in Fig. 6 and the time elapsedsince the coil was energized is the same as that illustrated in Fig. 6.The flexible armature in Fig. 7, however, has already been drawn towardthe pole face in the area nearest to the armature axis. Nevertheless,the flux build-up has not been sufficient to draw the armature 4 awayfrom its backstop 37. The result, as clearly illustrated, is a markedbowing or flexing of the armature as it is pulled against the springload. It will now be understood that as the armature 4 in Fig. 7 movesthrough its arc of travel, the area of contact between the armature 4and the pole face, or separator 3, will advance progressively from thepoint on the outer pole face nearest to the armature hinge axis to thepoint on the outer pole face furthest from the armature hinge axis. Thearmature l will thus close with a rolling like motion across the polefaces until it comes to rest in the closed or operated position at whichpoint it will again become straight or flat as it was prior to theenergizing of the coil. The effect of the bending action of the armatureis to change the traditional relationships between the armature gap andthe armature travel, between the armature travel and the effective pulland between the pull and load lever arms. All of these effects may beutilized to provide a relay armature wherein the pull characteristicsclosely approximate the load characteristics.

It is well known that flux concentration in the air gap of a relay isinversely proportional to the length of the air gap. Thus, initially,the effective center of pull of the magnet will be relatively close tothe armature hinge axis in the unoperated position whether the armaturebe rigid or flexible. With the flexible armature, however, the effectivecenter of pull of the magnet moves rapidly away from the axis as thearmature travels to its operated position. As a result thereof,advantage is taken, initially, of having a short lever arm from hingepoint to pull center, permitting a high lever ratio to the load arm,while retaining the final advantage of a low lever ratio to the loadarm.

The magnitude of these new relationship is readily apparent froms Figs.9 and 10. The family of load curves in Fig. 9 is fairly representativefor any multicontact wire spring relay. The lower pull curve is thatplotted from the operation of a Nil-ampere turn wire spring relay with afiat E type core structure and a rigid armature. It will be noted thatthe curve is generally concave upward, that it intersects the maximumload curve approximately at the critical point, the break in the loadcurve nearest to the armature closed position, and that in approachingthe closed position or point of zero armature travel, the pull in gramsincreases sharply to extremely high values. The essentially exponentialcharacteristic of the curve results from the basic designcharacteristics of the conventional relay whereby a small force isexerted at large air gaps and a steadily increasing force is exerted asthe armature approaches the core. The lever arm over which the forceoperates also remains substantially constant throughout the travel ofthe armature. Consequently, as may be readily seen from Fig. 9, the pullcurve does not approximate the load curve.

The top curve in Fig. 9 has been plotted from values observed in theoperation of a 200-ampere turn wire spring relay with a type of magneticstructure disclosed herein, i. e., a cup-type core with integralconcentric pole faces and a flexible armature. It will be noted that thetop curve is generally concave downward, that it clears the criticalpoint of the load curve by a comfortable margin, and that in approachingthe closed position or point of zero armature travel, there is noradical increase in the grams of pull. The areas partially enclosed bythe two curves may be literally referred to as areas of improvement. Thehatched area to the right of the intersection of the two curvesillustrates the significant increase in pull force throughout the majorpart of armature travel that is attained in a relay which embodiesfeatures of the invention. The hatched area to the left of theintersection of the two pull curves illustrates the marked advantage ofthe improved relay in maintaining the pull force at a compaartively lowlevel near the end of the armature travel. The distinct advantage gainedthereby is that the application of the operating force to the load ismore analogous to a rocking chair squeeze than to a hammer blow. Thisadvantage is reflected in a lower final armature velocity and in thereduced shock of armature closing impact. Furthermore, these advantagesare attained with faster operate and release times per ampere turnvalue. The area between the two curves to the right of theirintersection illustrates the increased pull force which results infaster operate time for a relay embodying features of the invention. Thearea between the two curves to the left of their intersectionillustrates the excessive magnetic flux which must decay upondeenergization of the conventional relay prior to armature release.

Hence, the area to the left of the intersection of the two a r curves isillustrative of the faster release time of a relay embodying features ofthe invention.

The pull curves of Fig. 9 illustrate clearly the broad area ofimprovement in pull curve characteristics attained in a relay embodyingboth the cup-type concentric pole face core and the flexible armature.Reference to Fig. 10 will be of assistance in pointing out thecontribution of the flexible armature feature itself in improving pullcurve characteristics. The dotted line represents a multiple contactwire spring relay load curve. The family of pull curves has been plottedfrom data observed from the operation of a relay embodying the featuresof the invention disclosed herein. The variation in the three pullcurves represents the effect of varying the thickness and hence theflexibility of the armature. The curve which reaches a peak pull ofapproximately 1140 grams was plotted from the operation of a relay withan armature thickness of .040 inch. A peak pull of approximately 950grams was reached with an armature of .036

inch thickness and a peak pull of approximately 850 grams was obtainedwith an armature of .028 inch thickness. In each case, the material ofthe armature was cold rolled steel. It is apparent that a primarycontribution of the flexible armature is in limiting the excessive pullexperienced toward the end of armature travel with a stiff orconventional armature, or, expressed in terms of the pull curve, thecontribution of the flexible armature is in flattening the curve.Moreover, it is apparent that varying degrees of armature flexibilitycan be used in order to shape or tailor the pull curve to conform to theparticular load curve resulting from any given relay design.

The type of novel core structure disclosed herein also lends itself tomodifications by which, in particular design situations, the pull curveof a relay can be made to conform more closely to the load curve than isthe case in relays known in the art heretofore. It has been found thatvarying the pole face area by changing the overall shape or size of thebasic design, or by means of notches or pole face apertures, has amarked effect on the configuration of the pull curve. Further tailoringof the pull curve can be efiected in a relay constructed according tofeatures disclosed herein by varying the initial heel gap or spacebetween the armature and that section of the pole face closest to thearmature axis. This varia tion can be readily obtained by appropriatepositioning of the level or plane of the armature hinge axis above theplane of the pole faces to the point at which the desired effect inshaping the pull curve is attained.

Still further tailoring or shaping of the pull curve is possible, forexample, by controlling the stiffness of the armature linearly ortransversely, or both, with ribs or bent over sections, and by chokingor redistributing the useful flux paths in the armature or core throughalteration of' the cross sectional area of magnetic material in any ofnumerous places in the relay.

While the exemplary form of core structure 1, illustrated in Pig. 5, maybe particularly well adapted to conform to the shape and spacelimitations and design requirements of a given relay installation, it isnot to be interpreted as limiting the scope of the invention to theparticular embodiment presented. For example, a number of common novelfeatures are present in the magnetic relay structure illustrated in Fig.8. There it will be observed that While a cross section of the structurepre sents essentially the same view would a cross section of the type ofcore and coil illustrated in 5, the rangement in Fig. 8 involves a corestructure that is straight rather than annular, a design which may bedescribed as a deep W type. While the greater part of the coil issubstantially enclosed in the cup-type recesses of the core, the ends ofthe elliptical coil are outside the core. 7

figuration of-the armature slots 4A as illustrated. Again it will benoted that the armature 4 is rotated about a pivot axis that islaterally spaced from the core It. While differing substantially fromthe arrangement of the embodiment of the invention illustrated in Fi 5,the device illustrated in Fig. 8 nevertheless retains the advantages ofthe features of the invention as discussed hereinabove.

Still further flexibility of mechanical design is possible in thecup-type core illustrated in Fig. 5. For example, the core need notnecessarily be round but could be square, rectangular, elliptical, or ofother convenient shape in order to match available space. Nor it isnecessary that the core structure be drawn ircm a single piece. Withinthe same overall dimensions, greater winding space and a larger centralpole face area may be ob t..ned by using a central shouldered stud, orpost, or other fabricated construction instead of an integral drawncenter. The head of such a post could be enlarged or not depending uponthe design requirements for the size of the center pole face.

All of the above embodiments of the features disclosed herein, andvarious modifications which may be suggested by the above discussions,may be used toward the end of exercising closer control over theelectromagnetic behavior of a relay or similar magnetic device in orderto achieve desired operating characteristics.

What is claimed is:

l. An electromagnetic device comprising a magnetic core having an innerpole-piece and an outer pole-piece encompassing and spaced from saidinner pole-piece, an energizing coil for said core, an armatureextending across and spaced from opposite portions of said outerpole-piece, said armature having an intermediate portion injuxtaposition to said inner pole-piece, and means mounting said armatureadiacent one of said opposite portions for rocking movement relative tosaid polepieces, said armature being sufficiently flexible to bowconcave upwardly with respect to said pole-pieces when said coil isenergized, thereby causing contact between said armature and saidpole-pieces to be effected progresively across said pole-pieces.

2. An electromagnetic device comprising a magnetic core having spacedinner and outer pole faces, an energizing coil for said core, anarmature, and means mounting said armature for pivotal movement about anaxis adjacent and outside of a portion of the outer pole face, saidarmature extending from said mounting means over said portion of saidouter pole face and over said inner pole face to an opposite portion ofsaid outer pole face, said armature being substantially flat butsufficiently flexible to bow concave upwardly with respect to said polefaces when said coil is energized, thereby causing contact between saidarmature and said pole faces to be effected progressively across saidpole faces from a point on said outer pole face relatively near saidmounting means to a point on said outer pole face relatively distantfrom said mounting means.

3. An electromagnetic device comprising a magnetic core having coaxialinner and outer pole faces, an energizing coil for said core, a supportlaterally beyond said outer pole face, and an armature mounted by saisupport, said armature extending over and in spaced relation to saidinner pole face and diametrically opposite portions of said outer poleface, said armature being substantially flat but sufficiently flexibleto bow concave upwardly with respect to said pole faces when said coilis energized, thereby causing contact between said armature and saidpole faces to be effected progressively across said pole faces.

4. An electromagnetic device comprisiu' a magnetic core having acup-shaped outer pole-piece and an inner pole-piece coaxial with saidouter pole-piece, an energizin. coil for said core, said pole-pieceshaving substantially coplanar pole faces, a flat armature overlying andspaced from said pole faces and of transverse dimensions at least aslarge as those of the outer pole face, and means mounting said armaturefor rocking movement about an axis laterally beyond said outer poleface, said armature being sufliciently flexible to enable the area ofcontact between said armature and said pole faces to increaseprogressively, after said coil is energized, from a point on saidarmature relatively close to said mounting means to a point on saidarmature relatively distant from said mounting means.

5. An electromagnetic device comprising a magnetic core having an innerpole-piece and an outer pole-piece encompassing and spaced from saidinner pole-piece, an energizing coil for said core, an armatureextending across and spaced from opposite portions of said outerpole-piece and having an intermediate portion in juxtaposition to saidinner pole-piece, and means mounting said armature in normally inclinedrelation to said polepieces and for rocking movement about an axislaterally beyond said outer pole-piece, said armature being sufficientlyflexible to bow concave upwardly with respect to said pole-pieces aftersaid coil is energized, thereby causing contact between said armatureand said polepieces to be effected progressively across said pole-piecesfrom a point on said outer pole-piece relatively near to said mountingmeans to a point on said outer pole-piece relatively distant from saidmounting means.

6. An electromagnetic device comprising a magnetic core having a pair ofpole-pieces terminating in spaced, coaxial, substantially coplanar polefaces, an energizing coil for said core, a flat, polygonal armatureoverlying said pole faces and extending beyond diametrically oppositeareas of the outer pole face and in spaced relation thereto, and meansmounting said armature adjacent one of its edges for rocking movementrelative to said pole faces, said armature being normally inclinedrelative to the plane of said pole faces, said armature beingsufficiently flexible to bow concave upwardly with respect to saidpole-pieces after said coil is energized, thereby causing the area ofcontact between said armature and said pole-pieces to be increasedprogressively across said polepieces from a point on said outerpole-piece relatively near to said mounting means to a point on saidouter pole-piece relatively distant from said mounting means.

7. An electromagnetic device comprising a magnetic core having a pair ofpole faces, an energizing coil for said core, a substantially fiatarmature extending across said pole faces in spaced relation thereto,the spacing between said armature and one of said pole faces beinggreater than that between said armature and the other of said polefaces, and means mounting said armature adjacent one of said pole facesfor rocking movement with respect to said pole faces, said armaturebeing sufliciently flexible to bow concave upwardly relative to saidfaces when said coil is energized thereby causing the contact areabetween said armature and said pole faces to expand progressively acrosssaid pole faces starting at a point relatively near to said mountingmeans.

8. An electromagnetic device comprising a magnetic core having spacedinner and outer pole-pieces, an energizing coil for said core, asubstantially flat armature having an intermediate portion injuxtaposition to said inner pole-piece and a pair of outer portions injuxtaposition to areas of said outer pole-piece on opposite sides ofsaid inner pole-piece, the spacings between said outer portions and saidareas respectively being different, and means mounting said armature forrocking movment about an axis laterially beyond one of said areas, saidarmature being sufficiently flexible to bow concave upward relatively tosaid pole-pieces when said coil is energized thereby to cause contactbetween said armature and said pole-pieces to be effected progressivelyacross said pole-pieces from a point on said outer polepiece relativelynear to said mounting means to a point on said outer pole-piecerelatively distant from said mounting means.

9. An electromagnetic device comprising a magnetic core having a pair ofcoaxial, substantially copolanar pole faces, an energizing coil for saidcore, an armature having a substantially flat portion extending betweenopposite areas of the outer pole face and overlying the inner pole face,means mounting said armature for rocking movement about an axislaterally beyond said outer pole face, and normally in inclined relationto said pole faces, said armature being flexible whereby to bow concaveupward relative to said pole faces when said coil is energized andthereby to cause the contact area between said armature and said polefaces to expand progressively across said pole faces starting at a pointrelatively near to said mounting means as said armature is being drawntoward said pole faces.

10. An electromagnetic device comprising a core having a cup-shapedouter pole-piece terminating in an annular outwardly extending flangedefining an outer pole face, said core having also an integral innerpole-piece terminating in a circular pole face substantially coplanarwith said outer pole face, an energizing coil for said core, a supportto one side of said flange, and a substantially flat armature mounted bysaid support for rocking movement relative to said pole faces, saidarmature overlying said pole faces, extending across said outer poleface and being normally inclined with respect to said pole faces, andsaid armature being sufliciently flexible to bow concave upward relativeto said pole faces when said coil is energized thereby to cause contactbetween said armature and said pole-pieces to be effected progressivelyacross said pole-pieces from a point on said outer pole-piece relativelynear to said mounting means, across the inner pole-piece and thence to apoint on said outer pole-piece substantially diametrically placed fromthe point on said inner pole-piece where contact with said armaturecommenced.

References Cited in the file of this patent UNITED STATES PATENTS2,382,664 Ray Aug. 14, 1945 2,442,016 Poole May 25, 1948 2,498,702Nahman Feb. 28, 1950

